The oldest known type of non-stick cookware is oil-seasoned steel or cast iron cookware. Such cookware is very effective, and has a tough, abrasion resistant surface. Unfortunately, seasoned steel or cast iron cookware is vulnerable to damage by rusting of the substrate metal, and must be handled and particularly cleaned with care to avoid damaging the cookware surface. The critical carbonized surface layer can be lost or damaged if the cookware is allowed to sit in water containing dissolved oxygen for even a few hours.
Another problem with seasoned steel or cast iron cookware is that iron can readily be leached from the cookware's surface by food acids, as are found in tomato sauce for example. Thus, depending on one's food preferences, it is possible to get a dose of iron from food cooked in seasoned iron-based cookware which is well above the daily recommended dose, which can have negative health effects for some people.
Another problem with seasoned steel or cast iron cookware is that reaction products of edible fats and oils (the source of the carbonaceous cooking surface), which are chemically modified by the process of seasoning the cookware, can sometimes escape back into food before the carbonization process is complete. Some intermediate compounds which are produced as fats and oils are chemically converted to the carbonized non-stick cooking surface are known carcinogens (such as trans-fatty acids and peroxidized fatty acids).
Another traditional material for foodware is copper. Copper foodware has excellent heat transfer properties, but is much softer than seasoned cast iron or stainless steel, for example. This makes it delicate and prone to scratching. It is also prone to surface oxidation and various reactions with sulfur compounds in food that lead to tarnishing. Similar to cast iron and steel foodware, copper foodware can introduce copper ions into food at fairly high levels compared to dietary requirements for copper. Copper can be polished to an attractive reflective surface finish, but requires constant work to maintain this surface finish. As a result of the difficulty of keeping copper foodware looking good, a large fraction of copper foodware in modern kitchens today is more for show than for cooking.
The difficulty of keeping copper foodware looking good implies that another approach involving a tougher metal which is better able to hold a shine than copper is desirable. One prior art solution for this is stainless steel. Stainless steel foodware is not normally seasoned with oil in the same way as cast iron or steel cookware. Food tends to stick to stainless steel much more than to seasoned cast iron. However, stainless steel foodware is strong and tough, and holds a finish very well (much better than copper). Furthermore, stainless steel foodware can be soaked in strong cleaning solutions, which makes it more clean-able and more convenient to maintain than seasoned cast iron.
Stainless steel foodware holds its shine much better than copper, but can still be scratched by stainless steel utensils, and it can be pitted by salt water (especially hot brine). Stainless steel foodware can also leach ions into food, especially iron, chromium, manganese, and nickel. Although the actual dose of ions from stainless steel is normally quite low, it is still a concern for some people. (Nickel in particular can leach from some grades of stainless steel at levels that are well above the recommended dietary allowance for nickel.)
Aluminum foodware has the best heat transfer properties per unit weight of any commonly available type of prior art foodware. Unfortunately, aluminum foodware is also prone to leaching of aluminum ions into food, in the form of food acid aluminum salts. Such food acid aluminum salts are much more able to enter the bloodstream than common inorganic forms of alumina (as in bauxite) or aluminosilicates (found in clays for example). Some researchers have expressed concern that blood-borne aluminum can accumulate in the brain, and may increase the risk of Alzheimer's disease.
One way to reduce the amount of aluminum getting into food from aluminum foodware is to coat the aluminum. Anodized aluminum is coated with aluminum oxide to a greater depth than is the case for untreated aluminum objects. (Aluminum that has been exposed to air is always coated with a thin layer of oxide; the oxide is much harder and more stable than the aluminum, and passivates the surface to some extent.) Anodized aluminum is much harder on the surface than untreated, air-oxidized aluminum surfaces because the oxide layer is thicker. Anodized aluminum is also far more resistant to leaching of aluminum ions by food acids than untreated, air-oxidized aluminum foodware. The anodized aluminum surface is not good for release of food during cooking, though; oil must generally be used to prevent foods from sticking.
Anodized aluminum foodware surfaces always contain microcracks, due in part to the difference of thermal expansion rate of aluminum versus aluminum oxide, which leads to thermomechanical stress as the foodware (especially cookware) temperature is changed. These cracks are sites for corrosion, and leach aluminum into food. These cracks can also grow, and yet remain undetectable to the naked eye, so that anodized aluminum foodware can leach more aluminum as it ages.
Anodized aluminum is not dishwasher safe. Typical automatic dishwasher detergents, dispersants, and wetting agents discolor and/or corrode anodized surfaces. Anodized aluminum foodware also can be discolored if oily drips get cooked onto the surface and then get hot enough to be carbonized. Such stains are difficult to remove without changing the appearance of the foodware in the region where the oil drip has been cooked onto the foodware, because the carbonized oil stain must be abraded off.
Another way that aluminum foodware can be modified to prevent leaching of aluminum ions into food is via thermal spraying (flame spray, plasma spray, or high velocity oxidizing flame, known commonly as HVOF spray) the inside of the foodware. In thermal spray methods, a hot gas or plasma is used to melt (or partially melt) a stream of solid particles, which are impinged against a relatively cold metal surface (in this case the aluminum substrate foodware) at a sufficient velocity to adhere to the surface. (The impinging molten particles from the plasma spray also cause localized melting of the aluminum surface, which can improve adhesion.)
Thermal spray methods cannot produce a smooth, specular (mirror-finish) surface. The rough surfaces produced are ideal for anchoring a perfluorocarbon layer, but if untreated, are generally prone to food sticking. Look Manufacturing Company's aluminum-core cookware, which are plasma sprayed with stainless steel, are examples of this approach. The resultant surface is very tough, but is not good for release of foods during cooking. (This type of surface does make an excellent substrate for application of perfluorocarbon-based release coatings, though.) Scan-Pan Inc. sells plasma sprayed aluminum cookware that is sprayed with titanium nitride (TiN). This cookware is much better for release of foods during cooking than otherwise similar cookware (in terms of porosity and surface roughness) which has been sprayed with stainless steel, for example. Scan-Pan's TiN surface coating is quite uneven, though the surface roughness is essentially devoid of sharp edges. The TiN is partially decomposed by the plasma's extreme temperature, so that the surface also contains metallic titanium in addition to titanium nitride.
Perfluorocarbon polymers, such as PTFE (polytetrafluoroethylene, "Teflon.TM." for example), have been used as non-stick coatings for cookware since the 1960's. The earliest technologies for applying such coatings over relatively smooth aluminum cookware produced very fragile cooking surfaces. Since then, these coatings have been improved by incorporating various reinforcing ceramics into the perfluorocarbon polymer layer, as in U.S. Pat. No. 4,795,777 or DuPont's U.S. Pat. Nos. 3,970,627 and 4,123,401 covering their Ironstone.TM. cookware (in which mica reinforces the perfluorocarbon layer).
Recent improvements of perfluorocarbon-coated cookware have involved anchoring the PTFE or perfluorocarbon polymer layer more durably to the cookware surface via surface roughening. Some of the most advanced techniques involve infiltration of perfluorocarbon polymers or polymer precursors into strong, porous surface layers on the cookware surface. Such rough and/or porous anchoring layers can be applied to the aluminum cookware by several methods, including thermal spray techniques, powder metallurgy, anodizing (see for example U.S. Pat. No. 5,545,439), and acid or base etching.
When surface roughening is applied to cookware that is subsequently coated with perfluorocarbon polymer release agents, the resultant cookware is not smooth on the cooking surface. The surfaces of such cookware usually contain features that deviate from the average local plane of the surface by more than 10 microns, and usually more than this; see for example U.S. Pat. No. 5,71 1,995 which describes advantages of deliberately rough cooking surfaces for perfluorocarbon-coated cookware. The surface energy densities of the release coatings are so low, however, that excellent food release is attained in spite of the surface roughness. Such coated cookware is designed so that if the surface is scratched, only the (abrasion-resistant) peaks of the substrate cooking surface are exposed by normal abrasion during cooking; most of the cookware surface remains protected by perfluorocarbon polymer release agents (which reside in the surface pores and depressions).
Perfluorocarbon polymers do not release ions into food unless the perfluorocarbon layer is damaged. Perfluorocarbon-coated cookware is easily damaged if hard cooking implements, such as metal utensils, are used to stir or manipulate food while cooking in them. The resultant flakes of perfluorocarbon coating can get into food. Although these perfluorocarbon flakes are not known to cause any health problems, many people object to the thought of perfluorocarbon particles getting into food. Cookware which is based on impregnation of the release coating into a hard, porous surface are somewhat more abrasion resistant than PTFE over smooth aluminum, but will still release perfluorocarbon particles into food if metal implements are used while cooking in them.
Another problem with perfluorocarbon-coated cookware is that, if it is overheated, the polymer layer will be damaged, and can release toxic fumes. (This should not happen in ordinary cooking, but it does mean that the cookware can be accidentally ruined quite readily.)
Another problem with perfluorocarbon-coated foodware (including especially, but not exclusively cookware) is that extremely hydrophobic foodware surfaces composed of fluoropolymers are also not wetted easily by cooking oil, so cooking oil "beads up" in the bottom of the foodware, which is not desirable. The perfect non-stick foodware surface in terms of surface chemistry would be wetted by olive oil very well, but be as hydrophobic as possible towards water-based dispersions and/or solutions containing starch, protein, and/or sugar.
There are a variety of other high-temperature stable polymers used in cookware besides the perfluorocarbon polymers. Examples include silicone resins and polyimides, for example. These types of polymers are mainly limited to ovenware, because their thermo-oxidative stability is not good enough to stand up to the much higher temperatures used in stovetop cooking. As with the perfluorocarbons, these ovenware coatings are easily scratched by metal culinary tools.
All the polymers described above can in principle be used for foodware items besides cookware, though they are more prone to be damaged than similar foodware made of stainless steel, for example.
Many types of ceramic coatings and/or foodware surfaces are known in the prior art. Most are based on oxides or mixed oxides (aluminates, aluminosilicates, and silicates are mixed oxides) that are melted or fired (which involves partial melting). Oxide and mixed oxide surfaces are relatively hydrophilic in the sense that water droplets residing on a clean oxide surface will wet the surface. (Wetting the surface means that the contact angle between the liquid/vapor interface and the liquid/solid interface at the leading edge of a droplet sitting on a solid surface is less than 90 degrees; good wetting implies that the contact angle is less than 10 degrees. See FIG. 1 for an illustration of contact angle.)
Foods sticking to foodware is mainly a problem for cookware. Other types of foodware, though, such as serving bowls and trays of the prior art can also be very hard to clean after various foods dry out on them (which should never happen in a well-managed kitchen, yet it does happen in reality). We shall use the term "solidify" in regard to foods cooking, dehydrating, or fermenting in such a way that an initially fluid food is converted to a solid. Adhesion of solidified foods to a foodware surface is dependent on:
1. the actual surface area of the foodware per unit of apparent surface area (which depends on microscopic surface roughness). PA1 2. the fraction of foodware surface area which is wetted by the food (which depends on how much of the interfacial area between the food and foodware is occupied by gas bubbles; poor wetting of the foodware by the food increases bubble coverage of the interface, reducing interfacial adhesion). When foodware is coated by a highly hydrophobic coating, such as PTFE, the high contact angle of semisolid foods tends to cause more gas bubbles to be trapped at the food/foodware interface, reducing adhesion proportionately. PA1 3. the interfacial adhesion between the solidified food and the foodware. This adhesion operates over the true interfacial area between food and foodware, after compensating for roughness and gas bubbles. Interfacial adhesion is a result of intermolecular forces, which are strongest between highly polar groups, such as hydroxyl and amide groups in particular. Most foods contain numerous hydroxyl groups and proteins contain numerous amides, and so foods tend to stick more tenaciously to foodware surfaces containing hydroxyl groups. PA1 4. The strength of the solidified food. If the solidified food is very weak, it will be easy to clean regardless how strong the molecular-level adhesion of the solidified food to the surface. PA1 1. Evaporation is the lowest energy gas/plasma deposited PVD coating process. Atoms or molecules arrive at the surface with approximately the thermal energy they have when they evaporate, less than 2 electron volts. Few charged species are usually formed or deposited in evaporated coatings. Evaporation works best for depositing layers of pure metal, for which the chamber pressure is determined by the vapor pressure of the metal, and can be as high as 10.sup.-3 torr. Evaporation can be from a liquid metal crucible, or via evaporation of a wire feed with an electron beam. It is very difficult, perhaps impossible to achieve efficient deposition of a metal nitride by reactive evaporation. In reactive evaporation, a gas which reacts with the metal vapor is allowed into the chamber, at a partial pressure comparable to that of the metal vapor. A complication of reactive evaporation is that as the pressure in the chamber increases, molecular clusters form in the gas phase, which does not lead to the desired low porosity, dense film on the foodware or other workpiece to be coated. Therefore, reactive evaporation is usually applied at reactive gas partial pressures no higher than 10.sup.-5 torr. It is possible to efficiently deposit a mixed metal/metal nitride film via reactive evaporation (this is an example of a partially nitrided PVD coating). Adhesion is often problematic with evaporated coatings; they are far less resistant to contamination on the workpiece surface than alternative PVD processes in which the arriving deposited atomic-scale species bring more energy to the surface per unit of material deposited. PA1 2. Sputtering is driven by a plasma of non-reactive atoms. Argon is usually used, but helium, neon, and xenon could all be used. To deposit nitrides or oxides (for example), a reactive gas is also introduced (nitrogen or oxygen), but the partial pressure of the reactive gas is normally less than the partial pressure of argon and/or other non-reactive gases. (This process, reactive sputtering, can deposit extremely smooth layers of various nitrides and oxides.) The plasma is excited by a radio frequency oscillating magnetic field. The argon plasma evaporates a metal target, and the evaporated metal (atoms, molecules, and ions) streams away from the target. Magnetic fields can be applied at the target to trap ions, which speeds up evaporation of the metal target by trapping plasma ions near the target surface; this is known as magnetron sputtering. Some of the sputtered metal deposits on the workpiece, and a lot usually winds up on the chamber walls, because it can't be aimed. Most of the sputtered material is not charged when it reaches the workpiece surface, but a larger fraction of arriving atomic-scale species are charged than is the case in evaporation; typically 20% are ionized. Applying a voltage to the workpiece, or modifying the radio frequency or intensity driving the plasma can change the fraction of the deposited material arriving at the surface as ions. An important fact about sputtering is that the argon plasma (both argon atoms & ions) delivers far more energy to the workpiece surface than the incoming metal/metal nitride atoms, ions, and molecules. This constant bombardment, of the surface tends to minimize residual stresses and defects in the surface. One can think of sputtering as analogous on a molecular level to shot peaning, in which repeated impacts on a metal surface lead to increased toughness. All this unproductive (in terms of laying down a coating) expenditure of energy also means that sputtering is relatively energy inefficient compared to evaporation. It is also common to use energetic argon plasmas to bombard and clean the workpiece surface. Oxide layers can for example be removed by an argon plasma treatment. A very desirable feature of sputtering is that it can be used to deposit a very wide variety ceramics. For example, non-electrically conductive carbides, nitrides, and oxides can be used as targets in sputtering, so that a much wider range of ceramic coatings can be applied by sputtering compared to other PVD methods. It is vital to have pressure low enough so that most of the deposited material travels from the target to the workpiece without encountering another atom or molecule on the way. Such an atomic-scale specie is said to be "ballistic" in that it has most of the energy it was given by the sputtering process when it impacts the workpiece surface. Ballistic atomic species in sputtering typically have kinetic energy around 15-20 electron volts, about ten times as much as atomic scale species impacts in evaporated PVD coatings. The desirability of ballistic trajectories for sputtered atomic scale species implies that the mean free path of atoms in the chamber must be further than the separation distance between the target and workpiece, so it is important to keep this separation small (typically about 15 cm). For a typical sputtering application, this implies that the pressures during sputtering are around 10.sup.-6 torr at most, and more typically at .about.10.sup.-8 torr. The low pressures limit the rate of deposition; sputtering is therefore one of the slowest PVD process. PA1 3. Cathodic arc deposition is a PVD process in which metal is first ionized (by an electric discharge most commonly, but also sometimes by lasers, electron beams, or ion beams) and the positively charged ions are electrically attracted to the workpiece. A major complication is that small droplets, which may be charged, are also formed in the ablation of the target; these droplets are a major cause of coating defects. In reactive cathodic arc deposition there is a reactive gas (typically nitrogen or oxygen) present. Depending on the vacuum level, the ions attracted to the workpiece might be ballistic, or they may have numerous collisions in their trajectory from the target to the workpiece. The bias voltage between the workpiece and the metal ion plasma can vary widely; it is customary to start out with relatively high voltages, high vacuum, and no reactive gas present so as to imbed the incoming metal ions into the surface ballistically. Ballistic metal ions can also be used to clean a mildly contaminated surface by ablation. Ballistic metal ions cause localized mixing, leading to a "fuzzy" boundary between the primer metal (titanium or chromium typically) and the substrate workpiece, which inhibits interfacial failure. One particular type of cathodic arc deposition is the ion plating method described in U.S. Pat. No. 5,447,803. In ion plating, an initial primer layer of metal is applied to the workpiece using a bias voltage between 300-1500 volts typically, in a vacuum below 10.sup.-6 torr (typically 10.sup.-7 torr). The partial pressure of nitrogen in the coating chamber is then raised high enough that a metal ion is likely to encounter several nitrogen molecules in its trajectory from the target to the workpiece (typically 10.sup.-5 torr). After each such collision, even if there are chemical reactions, the metal-containing fragment generally remains positively charged and so continues on its trajectory towards the workpiece (because of the bias voltage). Ion plating works best when a particularly appropriate molecular ion is formed efficiently from the collisions of metal ions with a reactive gas. This is especially the case for titanium (which tends to form the TiN.sup.+ ion), and for zirconium (which tends to form the ZrN.sup.+ ion), with nitrogen as the reactive gas. Titanium and zirconium are particularly appropriate for ion plating because these elements readily form compounds with the metal in both the +3 oxidation state (such as TiN, ZrN), and the +4 oxidation state (such as TiO.sub.2, Ti.sub.3 N.sub.4, ZrO.sub.2, TiN.sup.+, and ZrN.sup.+). Magnetic fields can be used to steer the cathodic ion stream somewhat. PA1 4. Ion beams can be used as part of a PVD process, for example to ionize a target for cathodic arc, or in some cases may be applied directly to a workpiece. One particular example of an ion beam coating process is filtered cathodic arc, a technique known in the art. In this process, crude molecular ion plasma is "filtered" by being bent by magnetic fields. Only ions with the correct charge/mass ratio and velocity are bent by the selected angle. This technique allows for the separation of clusters and droplets from a stream of ions. The nearly pure stream of molecular ions produced by filtered cathodic arc (e.g., TiN.sup.+ and/or ZrN.sup.+) can properly be called an ion beam. Most ion beams, though, are based on either noble gases (e.g., argon) or reactive gases (e.g., oxygen or nitrogen). Such ion beams can be used in conjunction with a reactive carbon-containing gas to deposit diamond-like carbon (DLC). Oxygen or nitrogen ion beams can also be used to efficiently oxidize or nitride a metal film laid down by other. means. Another relevant example of a coating which can be applied directly by an ion beam is the coating formed by application of charged buckminsterfullerene (C.sub.60) to a workpiece. PA1 1. Improved scratch resistance can be attained via a layer of chromium or aluminum nitride, which is applied to a depth from 2-50 microns in particular areas of the foodware prone to scratching . These scratch resistant coatings can also serve as primer layers for other topcoat ceramic layers. PA1 2. Improved foodware of this invention incorporates gold-colored ceramic-coated foodware with improved color stability in which the color stability is enhanced by either: PA1 using oxygen or nitrogen plasma to stabilize the color after a PVD coating process, or PA1 zirconium nitride and/or zirconium-titanium nitride alloys are applied as the top visible ceramic layer on the foodware, determining its color and its resistance to discoloration. PA1 an optimized PVD process is used in which TiN.sub.x or TiN.sub.x -based alloys are deposited via cathodic arc deposition at a temperature between 375-450.degree. C.; or PA1 3. Ceramic-coated foodware with improved non-stick properties in which a hydrophobic ceramic such as diamond; diamond-like carbon; carbon-based alloys with boron, nitrogen, silicon, or titanium; boron nitride; zirconium carbide; or silicon carbide is applied as a smooth coating in a PVD or CVD process. PA1 4. Ceramic-coated foodware with improved heat transfer in which the surface touching food being cooked or chilled is composed of a high thermal conductivity ceramic, such as diamond, diamond-like carbon, silicon carbide, or aluminum nitride. PA1 5. Ceramic foodware with improved non-stick properties and improved thermal fatigue resistance in which a fired ceramic such as aluminum nitride or silicon carbide is used to form an item of foodware, such as a grille or a pan. The polished surface of such foodware is a very good cooking surface per se, or this surface can be further modified by PVD or CVD processes. PA1 6. Ceramic coated foodware in which a top clear layer of silicon nitride, alumina, or diamond-like carbon is applied for decorative effect, creating a "lacquered" surface appearance. PA1 7. Ceramic-coated metal foodware in which adhesion and corrosion resistance is enhanced via a first primer layer of chromium, and a second primer layer of chromium nitride, followed by a third layer of a nitride or carbide ceramic of the prior art. PA1 8. Ceramic-coated foodware (or a component of foodware, such as a pan but not the handle) in which a substrate foodware item composed of an aluminum alloy is plasma sprayed with chromium or a high chromium alloy. Said outer plasma-sprayed chromium layer may optionally be polished, electroplated with additional chromium, or PVD coated with additional chromium prior to application of a chromium nitride (CrN.sub.x) layer. After the CrN.sub.x layer is applied, the various PVD and CVD coatings of this invention may then (optionally) be applied over the CrN.sub.x primer layer. PA1 9. The improved foodware of this invention includes designs in which an anisotropic thermal core containing graphite is contained inside a metallic shell, similar to the cookware of U.S. Pat. No. 4,541,411, but improved by interlayering the graphite sheets with a soft, conductive metal. By combining graphite with other materials it is possible to achieve a selected thermal anisotropy between that of aluminum and that of pure graphite. Such improved foodware may optionally be coated by PVD or CVD ceramic coatings of this invention or the prior art. PA1 10. The improved foodware of this invention includes ceramic-coated designs in which the substrate foodware below the ceramic outer surface is composed of a carbon-carbon composite. PA1 11. The improved foodware of this invention includes ceramic-coated designs in which alternating layers of chromium/chromium nitride are interlayered to enhance the surface toughness of the foodware . PA1 more resistant to abrasive scrubbing than other decorative, shiny foodware (such as copper, brass, or stainless steel); PA1 more resistant to scratching and gouging than prior art metallic or TiN.sub.x -coated cookware of U.S. Pat. No. 5,447,803; PA1 better for release (non-stick) properties during cooking than prior art stainless steel, copper, aluminum, anodized aluminum, or ceramic-coated foodware, including the surface modified TiN.sub.x -coated cookware of Modes 2 or 3 of U.S. Pat. No. 5,447,803; PA1 better for retention of a highly polished decorative surface finish during cooking than prior art metallic foodware (such as copper, brass, or stainless steel) or the TiN.sub.x -coated cookware of U.S. Pat. No. 5,447,803; PA1 simpler surface stabilization (via ambient oxidation) of ZrN.sub.x surfaces compared to the TiN.sub.x surfaces of the cookware of U.S. Pat. No. 5,447,803; PA1 more resistant to leaching of ions into food from the foodware by food acids than prior art metallic foodware; PA1 more resistant to soaking, detergent washing and caustic automatic dishwasher cleaning solutions than aluminum, anodized aluminum, or oil-treated steel or cast iron foodware; PA1 more resistant to salt-catalyzed corrosion than stainless steel foodware; PA1 better spreading of cooking oil or other edible fats over the cooking surface of the foodware than perfluorocarbon-coated foodware. PA1 1. Highly corrosion resistant substrate foodware may be employed, which have outer surfaces that are more corrosion resistant than AISI 304 stainless steel, such as AISI 316 stainless steel, silicoaluminates, borosilicate or quartz glass, the glass-matrix composites of U.S. Pat. No. 4,265,968, stellite or other cobalt alloys, chromium or chromium alloys, monel, or titanium. PA1 2. Relatively thick primer layers of highly corrosion resistant metals or alloys may be employed to minimize the chance that pits can penetrate through both the primer and the coating. PA1 3. Multiple alternating layers of primer metal/ceramic/primer metal/ceramic, etc. may be employed; in most cases pinholes will not penetrate through several alternating layers. Provided that the primer metal/ceramic interface is not subject to corrosion in the presence of salty water, food acids, and/or oxygen, such foodware can be extremely corrosion resistant. PA1 4. Gradual interfacial layers may be generated via high-energy ion bombardment (as per U.S. Pat. Nos. 4,764,394 or 5,354,381 for example). Gradual interfacial layers are characterized by an interfacial zone rather than a sharp interfacial line between base metal and primer metal, and also between primer metal and ceramic coating; these gradual interfaces are usually free of pinholes and have excellent corrosion resistance. These gradual interfaces improve adhesion of plasma-applied ceramics greatly, and make it possible to get excellent adhesion without a metallic primer layer. PA1 5. Re-fixturing the foodware in the coating chamber between individual coating treatments tends to decrease the likelihood that individual pinholes penetrate through the entire coating layer; this may be applied either to the alternating layers of (3) above, or to repeated layers of one single ceramic or metal. PA1 6. Re-polishing the foodware between subsequent coating operations practically guarantees that pinholes will not line up. This method can, but need not, involve re-fixturing the foodware between coating, polishing, and re-coating. It is preferable from a cost viewpoint if the polishing can be performed in the coating chamber.
In comparing different surfaces with equivalent surface roughness, hydrophobic surfaces tend to have better food release properties (less adhesion between the food and the foodware) than hydrophilic surfaces. The only commercially significant prior art examples of highly hydrophobic surfaces on foodware are PTFE and other fluoropolymers, and silicone resins (used in bakeware, primarily).
The improved, coated foodware of this invention can be coated by PVD ("physical vapor deposition"), or CVD ("chemical vapor deposition"). PVD implies that the metallic portion of the coating composition goes into the coating chamber as a condensed phase, usually a solid or molten metal or metallic alloy, but in some few cases as a solid or liquid compound (such as an oxide, nitride, or carbide) containing metal atoms. In CVD, the metal atoms enter the coating chamber as a gas. The reactive species which react with the metal (if any) generally enter the coating chamber as a gas for both PVD and CVD processes.
Plasma plays a role in some CVD and PVD processes, to varying extents. Evaporation of metals (a PVD process) and atmospheric CVD do not involve plasma at all. CVD processes at reduced pressure also usually do not involve plasma, except for plasma-assisted CVD. Plasma is critical to both the sputtering process and cathodic arc deposition, the two most important PVD processes. Plasma-assisted CVD occurs at much lower temperatures than gas phase CVD; diamond films can be deposited via plasma assisted CVD at temperatures as low as 300.degree. C.
PVD can occur by several different processes. In all of these processes, it is desirable to coat the workpiece with atoms, molecules, and atomic/molecular ions (collectively, "atomic-scale species"), rather than microscopic molecular clusters or macroscopic droplets. All the PVD methods have the potential to deposit clusters or droplets as well as atomic-scale species. For all PVD coatings, the potential for clusters and droplets forming goes up with increasing pressure and decreasing mean free path of gas/plasma phase reactive species. Reactive PVD involves introducing a reactive gas into the chamber, which can lead to the formation of nitride or oxide coatings for example (using nitrogen & oxygen reactive gases). PVD includes the following specific methods (these methods are listed in order of increasing energy of the arriving atoms, molecules, or ions impinging on the surface to be coated):
These processes are known in the art. The evenness of the coating deposition in any of these processes is enhanced via repositioning or preferably rotating the foodware during the coating process. Surface polishing operations between two or more plasma coating treatments improves corrosion resistance, because it decreases the chances of a pinhole penetrating both coating layers.
Chemical vapor deposition (CVD) is also a well-developed process with many different variations. In general, gas phase CVD usually occurs at higher temperatures and higher pressures than PVD. Indeed, some CVD processes occur at atmospheric pressure. The most relevant CVD processes from the viewpoint of the present invention occur at reduced pressure, and are accelerated by a plasma (so they also occur at reduced temperature compared to gas-phase CVID processes); such processes are known as "plasma-assisted CVD".
Plasma-assisted CVD is a process that looks a lot like reactive sputtering in terms of equipment used, and methods of operation. However, there is no sputtering target, and the reactants (other than the substrate) enter the coating chamber as gases. The most important CVD process in terms of the present invention is the plasma-assisted CVD of diamond films. In this process, atomic hydrogen plays a critical role in allowing diamond to form (even though graphite is favored thermodynamically). Because diamond has such superior properties, there has been a very large amount of research interest in developing diamond films, and the field is by now well advanced. It is possible to grow CVD diamond on a variety of substrates, though freshly polished carbides and nitrides work particularly well.
Extremely smooth oxides, as may be deposited by PVD in particular, can have excellent non-stick properties even though they have hydrophilic surfaces that are easily wetted by sticky food solutions and/or suspensions of starches, sugars, and proteins. A prime example of this is contained in U.S. Pat. No. 5,447,803, which describes a process for stabilizing titanium nitride coatings on cookware via post-treatments in either hot nitrogen gas, or a nitrogen/oxygen mixture (with less O.sub.2 than air).
Insofar as U.S. Pat. No. 5,447,803 is the closest prior art to the present invention, we shall discuss it in detail below. U.S. Pat. No. 5,447,803 mentions PVD (physical vapor deposition) and CVD methods for applying the coatings, but with only one type of PVD process discussed in any detail, ion plating (a cathodic arc deposition process).
U.S. Pat. No. 5,447,803
The cookware of U.S. Pat. No. 5,447,803 were first coated with a primer layer of titanium metal .about.0. 02 micron, followed by reportedly up to 3 microns of titanium nitride (TiN.sub.x). This is actually pushing the practical limits for TiN.sub.x coatings, which are normally put down no thicker than 2 microns because they become prone to flaking off when put on the surface too thickly. This initial TiN.sub.x. coating was primarily composed of TiN, but reportedly contained metallic titanium as well (i.e., x&lt;1.0). Although U.S. Pat. No. 5,447,803 discloses titanium nitride-coated cookware, the claims that were allowed apply only to post-oxidized TiN.sub.x cookware. Claims 1 and 2 of U.S. Pat. No. 5,447,803 are limited to post-treated pans that were exposed to an atmosphere in which the ratio of oxygen to nitrogen was between 1:20 to 1: 5. Claims 3 and 4 are dependent on claims 1 and 2, and so are also limited to the same range of O.sub.2 :N.sub.2 ratio.
The patent text of U.S. Pat. No. 5,447,803 clearly describes three modes for TiN-coated pans. (The description makes it clear that the invention is only applicable to shallow pans.)
1. Mode 1 of U.S. Pat. No. 5,447,803 is application of TiN (PVD or CVD coating) over a titanium metal primer layer. Mode 1 is not described by any Claim of U.S. Pat. No. 5,447,803.
2. Mode 2 of U.S. Pat. No. 5,447,803 involves the surface chemical modification of Mode 1 coatings by post-treatment at atmospheric pressure and a temperature at or above 350 degrees C., in a gas mixture containing oxygen and nitrogen as the major components. Claim 1 limits the mole ratio of O.sub.2 :N.sub.2 to be between 1:20 to 1:5, yet the text of the patent leaves open the possibility of treatment via Mode 2 of TiN-coated pans in air (in which the O.sub.2 :N.sub.2 ratio is 1:4).
3. Mode 3 of U.S. Pat. No. 5,447,803 involves the surface chemical modification of Mode 1 coatings by post-treatment at a temperature at or above 350 degrees C., by an atmosphere consisting of essentially pure nitrogen.
Although U.S. Pat. No. 5,447,803 clearly describes all three modes summarized above, the claims of U.S. Pat. No. 5,447,803 do not read on Mode 1 or Mode 3 of U.S. Pat. No. 5,447,803. Therefore the technology described as Mode 1 and Mode 3 of U.S. Pat. No. 5,447,803 have been disclosed but not patented, and so are no longer patentable per se.
Based on the reported gold color and stoichiometry of the cookware of U.S. Pat. No. 5,447,803, it could not have contained a large amount of Ti.sub.3 N.sub.4 (Ti.sub.3 N.sub.4 is brown). Two different stabilizing methods are described in U.S. Pat. No. 5,447,803: controlled oxidation (Mode 2) and further nitriding (Mode 3).
In controlled oxidation (Mode 2 of U.S. Pat. No. 5,447,803), a surface layer is oxidized to titanium dioxide (TiO.sub.2), at a preferred temperature between 350-500.degree. C. The surface layer of TiO.sub.2 that forms is reportedly only about 0.02 microns thick, Since TiO.sub.2 occupies more volume than TiN or Ti, there is an expansion when the TiN.sub.x -coated surface is oxidized. This expansion puts the outermost surface of the coating into compressive stress, which tends to close surface microcracks and inhibit new cracks from forming. Because the protective TiO.sub.2 layer is so thin, it can be easily breached by sharp cooking or serving implements, or scratched through in normal service. This would defeat the color stabilization process of Mode 2 of U.S. Pat. No. 5,447,803, as the TiN.sub.x layer under any such scratches would tend to change color during cooking, leading to a pan displaying at least two different gold color tones.
When a TiN-coated pan is stabilized via Mode 3 of U.S. Pat. No. 5,447,803, it is heated in a nitrogen atmosphere at temperatures between 350.degree. to 600.degree.C. This converts some Ti metal to TiN (as proposed in the text of U.S. Pat. No. 5,447,803), but this is not the only process expected to occur under the reaction conditions described. When TiN.sub.x is heated to 350.degree. to 600.degree.C. in nitrogen, further nitriding of the surface will occur, producing a Ti.sub.3 N.sub.4, surface. Efficient conversion to Ti.sub.3 N.sub.4 was not recognized to have occurred by the authors of U.S. Pat. No. 5,447,803, though they did describe a deepening of the gold color tone, which is consistent with formation of a thin layer of Ti.sub.3 N.sub.4, of approximately 0.02 microns thickness (similar to the thickness of the TiO.sub.2 layers formed in Mode 2).
FIG. 12 of U.S. Pat. No. 5,447,803 also shows evidence that a new crystal form of TiN.sub.x, has appeared on the surface of Pan C (based on x-ray diffraction experiments). This was not noted by the authors of U.S. Pat. No. 5,447,803. As with Mode 2 stabilization (oxidation), there is an expansion when the TiN.sub.x -coated surface is further nitrided to Ti.sub.3 N.sub.4. This expansion puts the outermost surface of the coating into compressive stress, which tends to close surface microcracks and inhibit new cracks from forming. As with Mode 2 stabilization, the protective Ti.sub.3 N.sub.4 layer is thin, and it can be easily breached by sharp cooking or serving implements, or scratched through in normal service.
TiN.sub.x -coated cookware prepared by all three Modes of U.S. Pat. No. 5,447,803 performed well in experiments involving deliberate burning of foods in the pans. The color of the Mode 1 pan changed during cooking, developing discoloration that looked different in different places on the pan. No difference was noted in adhesion of burned foods between Mode 1 (TiN.sub.x) versus Mode 2 (TiO.sub.2), or Mode 3 (Ti.sub.3 N.sub.4) surfaces. Application of the Mode 2 oxidized surface did not change the appearance of the original substrate (Mode 1) pan, but did stabilize its appearance. A darkening of the gold color of the Mode 3 (Ti.sub.3 N.sub.4) coated pans versus the Mode 1 and Mode 2 pans was noted. The nitrided surfaces of Mode 3 pans appear darker due to the thin layer of Ti.sub.3 N.sub.4 that forms at the surface.
Both types of color-stabilized pans (Mode 2 and Mode 3) maintained their color after repeated cooking experiments. (These cooking experiments did not include deliberate efforts to test the scratch resistance of the pans). Note though, that even the Ti.sub.3 N.sub.4 coated pans will have a thin TiO.sub.2 layer after a relatively short exposure time to oxygen at cooking temperatures, which would tend to equalize the sticking behavior between all three types of pans (as was actually reported).